Skuttle Model 2001 and 2101 Automatic Humidistat
The Skuttle Model SEH-7100-000 electronic humidistat accommodates all Skuttle (and major other brands) central-system humidifiers.
While designed to operate automatically, it can also be installed for manual operation. Some of its features are:

Automatic adjustment of your home's relative humidity (RH) based on the outdoor temperature and your preferred level of comfort.

Solid-state components for safer, more reliable operation.

Out-of-the-way installation. Mounts on the return air duct; eliminates unsightly wall-mounting and the chore of fishing wires through walls.

Safety-conscious design. Does not intrude into the air duct; exposed materials and components are UV resistant.